1. Field of the Invention
The present invention generally relates to spinal fixation systems and the like. More particularly, an embodiment of the invention relates to a spinal implant system for correction, fixation, and stabilization of the human spine to allow the development of a solid spinal fusion.
2. Description of the Related Art
Spinal fixation, such as lumbar sacral fusion and the correction of spinal deformities such as scoliotic curves, is a well known and frequently used medical procedure. Pedicle, lateral, and oblique mounting devices may be used to secure corrective spinal instrumentation to a portion of the spine that has been selected to be fused by arthrodesis.
A spinal fixation system typically includes corrective spinal instrumentation that is attached to selected vertebrae of the spine by screws, hooks, and clamps. The corrective spinal instrumentation includes spinal rods or plates that are generally parallel to the patient""s back. The corrective spinal instrumentation may also include transverse connecting rods that extend between neighboring spinal rods. Spinal fixation systems are used to correct problems in the lumbar and thoracic portions of the spine, and are often installed posterior to the spine on opposite sides of the spinous process and adjacent to the transverse process.
Various types of screws, hooks, and clamps have been used for attaching corrective spinal instrumentation to selected portions of the patient""s spine. Examples of pedicle screws and other types of attachments are illustrated in U.S. Pat. Nos. 4,763,644; 4,805,602; 4,887,596; 4,950,269; and 5,129,388. Each of these patents is incorporated by reference as if fully set forth herein.
An eyebolt assembly of the TSRH(copyright) spinal system sold by Danek Medical Inc. is illustrated in FIG. 1. The eyebolt assembly 2 encircles spinal rod 4 such that assembly mass completely surrounds the spinal rod. The spinal rod must be inserted through the eyebolt, which rests within the yoke of spinal hook 8. The spinal hook attaches the spinal rod to a bony element of the spine. A nut 6 is threaded onto a post of the eyebolt assembly to fixably secure the rod within the yoke. The nut is tightened so that the assembly resists axial, torsional, and shear forces to inhibit motion of the spinal rod relative to the assembly in the directions indicated by the arrows in FIG. 1. Further details of the TSRH(copyright) spinal system are provided in the TSRH(copyright) Spinal Implant System Surgical Technique Manual and the TSRH(copyright) Crosslink Surgical Technique Manual. Both of these publications are available from Danek Medical Inc. and are incorporated by reference as if fully set forth herein.
Manual insertion of a spinal rod through the bores of a number of spaced-apart eyebolts within a surgical wound tends to be difficult. The bore axis of each eyebolt must be properly aligned along a common axis, which is difficult since the corrective procedure requires that the spinal rod initially be placed under stress to resist deforming forces of the spine. Therefore, the use of systems such as the TSRH(copyright) spinal system may require that a predetermined number of screws or hooks be pre-loaded onto the spinal rod in a particular order and spacing prior to the insertion of the spinal rod into the surgical wound. After insertion of the spinal system into the surgical wound, however, it is often necessary to add, delete, or reposition one or more hooks or screws. Before such modifications can be made, the spinal system typically must be removed from the surgical wound and at least partially disassembled.
To overcome such problems, some spinal fixation systems include xe2x80x9copen backxe2x80x9d hooks or screws to allow a spinal rod to be dropped into the open back of the hook or screw and secured within the open back by a separate component and a set screw. Such a system is illustrated in U.S. Pat. No. 5,102,412 to Rogozinski, which is incorporated by reference as if fully set forth herein. Such systems tend to be susceptible to fatigue stress failure and require assembly within the surgical wound. In addition, adding a hook or screw to the construct tends to require that the spinal rod first be repositioned. A further disadvantage of this approach is that component mass completely surrounds the spinal rod, resulting in an increase in the profile width of the device and greater impingement of the device upon the fusion mass. A low profile width is generally desired to minimize sinus formation and soft tissue irritation from hardware prominence.
U.S. Pat. No. 5,242,445 to Ashman relates to a xe2x80x9csplit eyeboltxe2x80x9d assembly for adding eyebolts to an assembled spinal fixation construction. Attaching the split eyebolt to a spinal rod requires a special crimping tool to crimp the split eyebolt over the rod. The crimping tool tends to be difficult to operate within the surgical wound. Furthermore, the threads of the opposing sides of the split eyebolt are often misaligned after crimping, making it difficult or impossible to thread a nut onto the split eyebolt. The split eyebolt also completely encircles the spinal rod thereby increasing the impingement of the construct upon the fusion mass.
It is therefore desirable that an improved spinal fixation system be derived that facilitates assembly and surgical implantation by allowing the spinal rod to be positioned within the surgical wound (a) after the fixation components (e.g., screws, hooks) have been implanted, (b) without modifying the fixation components, and (c) whereby fixation components may be subsequently added, deleted, and/or repositioned without disassembling the system.
In accordance with the present invention, a spinal fixation system is provided that largely eliminates or reduces the aforementioned disadvantages of conventional spinal fixation constructions. An embodiment of the invention relates to an implant system for fixation of the human spine that includes a spinal rod, a fixation component, a connector, and a fastener.
The connector may be used to connect the spinal rod to the fixation component and preferably includes a receiving end and a fastening end. The receiving end may contain a first arm and a second arm that together form a substantially U-shaped borehole into which the spinal rod may be axially positioned. The receiving end preferably surrounds only part of the spinal rod such that the unsurrounded portion of the spinal rod is exposed from the borehole. The exposed portion of the spinal rod may extend out of an open end of the U-shaped borehole. The spinal rod may be circular and preferably includes a cross-section having a circumferential portion. The receiving end of the connector preferably surrounds and engages greater than about xcfx80 radians and less than about 2xcfx80 radians of the circumferential portion.
The receiving end of the connector preferably acts as a xe2x80x9cpinch clampxe2x80x9d by exerting a clamping force on opposing sides of the spinal rod to secure the spinal rod within the borehole. The connector preferably contains a slot between the receiving end and the fastening end that enables the first arm and the second arm to be deflected relative to one another. The deflection of the arms allows the distance between a tip of the first arm and a tip of the second arm to be changed so that the spinal rod may be inserted through an open end of the U-shaped borehole that is defined between the tips of the arms.
The fixation component preferably includes a fixation device such as a bone screw or hook for engaging vertebrae of the thoracic or lumbar spine. The fixation component also preferably includes a body containing a cavity with an inner surface. The cavity is preferably sized to receive a portion of the connector. The connector is preferably partially disposed within the cavity such that at least a portion of the fastening end extends from the cavity, whereby the inner surface of the cavity engages an outer surface of the receiving end. The cavity of the body is preferably a tapered cavity that narrows in a direction from a first end of the cavity to a second end of the cavity. The tapered cavity preferably surrounds a portion of the receiving end and imparts a compressive force against the receiving end to secure the spinal rod within the borehole.
The fastener preferably engages both the body and the portion of the fastening end that extends from the cavity. The fastener may secure the connector and the fixation component together. The fastener is preferably a nut adapted to be threaded upon the fastening end. The fastener may be selectively tightened to allow an engagement between the connector and the spinal rod that may be overcome by the application of a distraction force to the connector. Rotation of the nut in a tightening direction about the fastening end preferably draws a portion of the receiving end through the tapered cavity, causing the inner surface of the cavity to compress the arms of the receiving end. In turn, the arms may exert a compressive force against the spinal rod to clamp it within the borehole. The magnitude of the compressive force against the spinal rod preferably varies as a function of the degree to which the nut is tightened. The open end of the U-shaped borehole preferably has a width that can be adjusted by tightening the fastener.
The fixation component may include a spacer located between the fastener and the spinal rod for laterally offsetting the fixation device a selected lateral distance from the spinal rod. The spacer may include a surface having a plurality of radially-spaced teeth. The fixation component may comprise a plurality of radially-spaced protrusions adapted to fit adjacent to the teeth on the surface of the spacer. The tightening of the nut preferably causes the spacer and the fixation component to become pressed together such that a complementary engagement between the teeth of the spacer and the protrusions of the fixation device is formed to inhibit rotation of the fixation device about the spacer.
The body may include a U-shaped yoke formed between a top section and a bottom section that each have an edge adjacent to the yoke. The tapered cavity preferably is formed between the top section and the bottom section and extends in a perpendicular direction relative to the U-shaped yoke. The fixation component is preferably adapted to pivot about the spinal rod in a substantially vertical plane. The edges of the top and bottom sections preferably contact the spinal rod during the pivoting of the fixation component to define the range of pivotal motion of the fixation component about the spinal rod. The edges are preferably curved in a direction away from the spinal rod to increase the range of pivotal motion of the fixation component.
The fixation component may include a transverse connector to maintain a fixed distance between the spinal rod and a neighboring spinal rod. The transverse connector may include a reduced section that has a width less than that of the body, allowing the reduced section to be more easily bent. The reduced section may be bent to shorten the lateral distance between the spinal rod and an adjacent spinal rod. The transverse connector may contain a beveled section between the body and the reduced section.
An advantage of the present invention relates to a fixation component that may be added to or deleted from a spinal fixation construct in a surgical wound without disassembling the construct.
Another advantage of the present invention relates to a spinal fixation system requiring minimal assembly within the surgical wound.
Yet another advantage of the present invention relates to a spinal fixation system having a relatively narrow profile width to reduce impingement upon the fusion mass.